Optical films such as Fabry Perot layers and photonic crystals have recently attracted attention as sensitive detectors of chemical or biological compounds. Prominent features of sensing with porous materials such as porous silicon, porous silicon oxide, or porous alumina are their small size, their stability, and their high sensitivity. Although the interaction between a porous optical film and an analyte does not involve a chemical reaction, it is able to detect the analyte optically. The detection is derived from changes to the optical band gap of the porous optical film, which is influenced by the difference of the refractive index between porous substrate and atmosphere. The optical characteristics of a porous optical film can be controlled during fabrication by adjusting the pore size and pore shape. Example applications of porous optical films include as a chemical sensor (WO 2006/044957), as a vapor sensor (US2008/252890; M. S. Salem et al. J. Appl. Phys. 2006, 100, 083520; B. H. King et al. Adv. Mater. 2007, 19, 4530-4534), a biosensor (US 2008/219615) and an acoustic sensor (WO 2008/086448). The films provide large surface areas, various surface chemistries and controllable optical and morphological properties including pore size, porosity, and refractive index allow the porous material to be tailored to specific sensing applications. The films can be formed to create various optical structures including Bragg mirrors, Fabry-Perot film, microcavities, and Rugate filters. Sinusoidal variations in the porous silicon refractive index with depth results in Rugate filters that provide a reflection stop band at a wavelength determined by the amplitude and period of the refractive index modulation. Infiltration of chemical vapors into the porous film shifts this spectral peak by increasing the refractive index of the porous layer thereby enabling transduction of ambient vapors by monitoring the magnitude and time evolution of the reflected stop band or Rugate peak wavelength. Films can be formed in various semiconductors and insulators, however, porous silicon is especially favored as it is inexpensive, readily oxidized and also provides biocompatibility. Most sensing has been conducted at ambient temperatures.
Susumu, et al, U.S. Published Application No. US2008/0252890 discloses a photonic sensor that includes a heating element. In Susuma, the heating element is used to refresh the sensor. It is also used as one of a number of methods to periodically modulate intensity or wavelength of the electromagnetic wave emitted from the sensor to permit detection of only the modulated electromagnetic wave, which helps distinguish a desired signal intensity peak from noise. The modulation of the signal wavelength by heat or other techniques in Susuma permits an analyzer to select only the modulated electromagnetic wave from others detected at the detector to discriminate the signal from electromagnetic wave noises at the detector. The modulation permits selection of the signal. Upon selection of the correct signal, the electromagnetic detector in Susumu utilizes a simple intensity measurement to determine the density of a substance introduced to the sensor. The intensity of the signal is compared with a reference intensity and the attenuation in the signal wavelength as compared to reference wavelength is used to determine the density.
Others have used temperature modulation during conductivity sensing as reported by Lee et al in “Temperature Modulation in Semiconductor Gas Sensing,” Sensors and Actuators B-Chemical, 60 (1999). The Lee article discusses temperature dependence of sensor conductance, along with transient and cyclic modulation techniques for improving sensitivity and selectivity of conductivity sensors in the analysis of single gases and multi-component gas mixtures. An illustrative example of these techniques is described by Nakata et al, in “Non-Linear Dynamic Responses of a Semiconductor Gas Sensor—Competition Effect on the Sensor Responses to Gaseous Mixtures,” Thin Solid Films, Volume 391, Issue 2, 16, pp 293-298 (July 2001). Nakata applied sinusoidal temperature cycles to semiconductor conductivity sensors and assessed the characteristic conductance-temperature profiles of light hydrocarbons. Others have also used programmed temperature profiles on arrays of metal oxide films to separate analyte behavior based on conductivity measurement. The conductivity changes are inspired in such approaches by high temperatures ranging from 200-600° C.